ESR spectra of Gd3+ probe have been studied in the range of ferroelectric phase transition T C =451 K in lead germanate Pb 5 Ge 3 O 11 . Measurements of ESR line position have shown that local order parameter behaves in accordance with the mean field theory approach in a broad (∼150 K) temperature interval below T C . Anomalous line width broadening observed around T C can be associated with a quasi-elastic central peak component of excitation spectrum, observed earlier using a light scattering experiment and attributed to static symmetry-breaking defects.

Earlier it has been shown that in the lithium heptagermanate Li 2 Ge 7 O 15 structure the chromium doping ions substitute the germanium hosts and create Cr 3+ -Li + pair centres. In the paper EPR and dielectric spectra of Li 2 Ge 7 O 15 :Cr 3+ crystal are studied in the temperature range 300 ÷ 500 K. The thermally activated reorientation of Cr 3+ -Li + pairs has been detected through EPR line width broadening and dielectric relaxation.

The dielectric constant ε and electron spin resonance spectra of Mn 2+ions have been studied in the range of the ferroelectric phase transition in Li 2 Ge 7 O 15 single crystals. Analysis of the experimental data permits one to estimate the critical exponents of dielectric susceptibility γ ∼ = 1.25, order parameter β ∼ = 0.32 and correlation length ν ∼ = 0.63. The values of the critical exponents determined in a fairly broad temperature interval around T c , are consistent with the theoretical predictions of the Ising model with a one-component non-polar ordering coordinate.

ESR spectra of Mn2+ probe have been studied in incommensurate rubidium zinc chloride monocrystals. It has been shown that temperature dependence of the resonance fields of ESR fine transition M S =3/2↔5/2 can be satisfactorily described based on the simple "local" model. ESR line position data confirm non-classical character of the Rb 2 ZnCl 4 critical properties, corresponding to 3D XY Heizenberg model.

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